Temperature-dependent Sex Determination - Hormones in TSD Systems

Hormones in TSD Systems

Synergism between temperature and hormones has also been identified in these systems. Administering estradiol at male-producing temperatures generates females that are physiologically identical to temperature-produced females. The reverse experiment, males produced at female temperatures, only occurs when a nonaromatizable testosterone or an aromatase inhibitor is administered, indicating that the enzyme responsible for conversion of testosterone to estradiol, aromatase, plays a role in female development. Nonetheless, the mechanisms for TSD are still relatively unknown, but in some ways, TSD resembles genetic sex determination (GSD), particularly in regards to the effects of aromatase in each process. In some fish species, aromatase is in both the ovaries of female organisms who underwent TSD and those who underwent GSD, with no less than 85% of the coding sequences of each aromatase being identical, showing that aromatase is not unique to TSD and suggesting that there must be another factor in addition to it that is also affecting TSD.

Interestingly, hormones and temperature show signs of acting in the same pathway, in that less hormone is required to produce a sexual shift as the incubation conditions near the pivotal temperature. It has been proposed that temperature acts on genes coding for such steroidogenic enzymes, and testing of homologous GSD pathways has provided a genic starting point. Yet, the genetic sexual determination pathway in TSD turtles is poorly understood and the controlling mechanism for male or female commitment has not been identified.

While sex hormones have been observed to be influenced by temperature, thus potentially altering sexual phenotypes, specific genes in the gonadal differentiation pathway display temperature influenced expression. In some species such important sex-determining genes as DMRT1 and those involved in the Wnt signalling pathway could potentially be implicated as genes which provide a mechanism (opening the door for selective forces) for the evolutionary development of TSD. While aromatase is involved in more processes than only TSD,it has also been shown to play a role in certain tumor development.